Field of the Invention
The present invention relates to an organic light-emitting display device for improving expression of low grayscales.
Discussion of the Related Art
Active matrix type organic light-emitting display devices include an organic light-emitting diode (referred to as “OLED” hereinafter) and have advantages of high response speed, high emission efficiency, high luminance and wide viewing angle. The OLED includes an organic compound layer formed between an anode and a cathode. The organic compound layer is composed of a hole injection layer (HIL), a hole transport layer (HTL), an emission layer (EML), an electron transport layer (ETL) and an electron injection layer (EIL). When driving voltages are applied to the anode and the cathode, holes that have passed through the HTL and electrons that have passed through the ETL move to the EML and generate excitons, resulting in generation of visible light from the EML.
Each pixel of an organic light-emitting display device includes a driving element which controls current flowing through an OLED. The driving element may be implemented as a thin film transistor (TFT). It is desirable that electrical characteristics of the driving element, such as threshold voltage and mobility, be equal across all pixels. However, electrical characteristics of driving TFTs of pixels are not uniform due to processing conditions, driving environment and the like. The driving element suffers from higher stress as driving time increases and the stress depends on a data voltage. The electrical characteristics of the driving element are affected by stress applied to the driving element. Accordingly, electrical characteristics of driving TFTs vary with time.
Methods for compensating for driving characteristic variation of a pixel in the organic light-emitting display device are divided into an internal compensation method and an external compensation method.
The internal compensation method automatically compensates for a threshold voltage variation in driving TFTs inside of pixel circuits. For internal compensation, current flowing through OLED needs to be determined irrespective of a threshold voltage of a corresponding driving TFT and thus a pixel circuit configuration becomes complicated. In addition, the internal compensation method has difficulty in compensating for mobility variation in driving TFTs.
The external compensation method compensates for a driving characteristic variation of each pixel by sensing electrical characteristics (threshold voltage, mobility and the like) of driving TFTs and modulating pixel data of an input image on the basis of the sensing result in a compensation circuit outside a display panel.
An external compensation circuit directly receives a sensing voltage from each pixel of the display panel through an REF line (or sensing line) connected to the pixel, converts the sensing voltage into digital sensing data to generate a sensing value and transmits the sensing value to a timing controller. The timing controller modulates digital video data of an input image on the basis of the sensing value to compensate for driving characteristic variation of the pixel.
To express a larger number of grayscales in a display device, a grayscale expansion method such as spatial dithering and frame rate control (FRC) can be applied. Such a grayscale expansion method can express higher-bit grayscale using a low-bit data driving circuit so as to achieve inexpensive display devices. Dithering can represent a larger number of grayscales than the number of bits of pixel data by dispersing decimal grayscale values below 1 to neighboring pixels. FRC disperses decimal grayscale values below 1 in the time domain to expand the number of grayscales. Dithering and FRC can be applied together.
When a grayscale expansion method is applied to the organic light-emitting display device, picture quality may be degraded such that grayscale representation is deteriorated or luminance is decreased.